Divide frequency of search string by corpus, taking care of cases like when a word is not in corpus.

Sort by answer in 4, the top n words are keyword.

###Why does this work?

The relative frequency in corpus string is a measure of how frequently the word occurs generally. Dividing search frequency word by
relative corpus frequency gets a measure of how frequently the word occurs relative to normal. Sorting on this gives us the keywords.

###The code

The code is longer than it needs to be make every step as clear as possible